Adenine and guanine nucleotides play important roles in regulating protein synthesis. It is proposed to study the relationship between high energy phosphate content and the rates of synthesis of total cardiac proteins and of two important classes of muscle proteins, the myofibrillar and the mitochondrial proteins. It is also proposed to study how certain proteins involved in the initiation of polypeptide synthesis may regulate synthesis of muscle-specific proteins. Using the intact beating fetal mouse heart in organ culture, total creatine (Cr) and purine nucleotide levels can be easily and reproducibly decreased (by oxygen and substrate deprivation) and increased (by supplying purine nucleosides, bases and Cr). Changes in ATP, GTO and creatine phosphate (CrP) contents and the NTP/NDP and CrP/Cr ratios will be correlated with changes in rates of protein synthesis, providing new information about the role of high energy phosphates in the normal and the deprived myocardium.